1. Field of the Invention
The present invention relates to golf clubs a set of golf clubs, and a method of making the same wherein each golf club has a club head structure having an improved position of the center of mass and is used for so-called through-the-green shots from a tee shot to a shot causing a ball to be on a putting green.
2. Prior Art
Conventional through-the-green golf clubs and those constituting a set of golf clubs have a club head structure wherein with respect to a central plane (A--A') which passes through the center of a shaft (11) and divides a club head (10) into front (hitting face) and rear portions, the position (17) of the center of the mass of a club head is located in the rear portion of the club head or substantially on the central plane, as shown in FIG. 3 (wood club head) and FIG. 4 (iron club head).
Some sets of golf clubs are constituted by a plurality of golf clubs such as woods whose depth of the center of gravity is increased as the club length is increased and, on the contrary irons whose depth of the center of gravity is decreased as the club length is increased. In this manner, the theoretical background about dynamic correlation between the depth of the center of gravity and dynamic flex of shaft due to the swing is left unclear.
The problems of conventional through-the-green golf clubs and those constituting a set of golf clubs will be described below with reference to the accompanying drawings.
As shown in FIGS. 6 and 7, the conventional golf clubs have a club head structure in which the position of the center of mass is as follows. More specifically, when the golf club is in a dynamic state immediately before the impact during golf swing, a club head end side portion is flexed forward (toward a ball flying direction) due to the inertia of the club head mass (as compared to a static state of the club), and impact occurs immediately thereafter.
The dynamic phenomenon due to the conventional golf club swing has been often reviewed in magazines as sequential photographs, and is well known. For example, a book "THE SEARCH FOR THE PERFECT SWING; by Alistair Cochran & John Stobbs, (1968) announces the measurement results shown in (FIG. 7). In FIG. 7, the swing was made by a professional golf player. Positive values indicated in units of inches in FIG. 7 represent that a club head is flexed forward, and negative indications of time (sec) represent time (sec) before impact.
Furthermore, the golf clubs require precise measurement of the dynamic phenomenon due to the inertia of the swing. For example, in a relatively easy test, a rod-like article serving as a club head is fixed to the distal end of a flexible shaft like a fishing rod, and this test tool is subjected to an ideal late-hit golf swing at a moderate speed. With this simulation, the dynamic phenomenon can be confirmed.
As is apparent from the above description, in a dynamic state immediately before impact during swing, a force for moving the club head backward acts on the conventional golf club in a dynamic state immediately before impact due to the elasticity of the shaft. The conventional club head structure has the position of the center of mass such that impact occurs at an instance while the backward dynamic flex of shaft continues. Therefore, the elasticity of the shaft acts as a vector opposite to the ball flying direction at impact when the club head contacts a ball. At an impact away from the center of the hitting face, the elasticity of the shaft acts as a negative force, and the directivity of the hit ball is easily lost. Therefore, it is known that as the elasticity of the shaft is increased, a distance of flight of the ball is decreased under the conditions wherein identical club heads are used with identical grips at identical club head speeds.
The dynamic phenomenon of the shaft causes the club head to accelerate during down swing, and a player feels as if a club head weight were decreased during an important short period of time during the down swing. This feeling is caused by a force flexing the club head end side forward against the elasticity of the shaft (i.e., inertia of the club head). This force corresponds to a force sensed as a force couple with respect to both hands of a player holding a golf club. For a right-hand player, impact occurs at an instance his right hand holds a golf club with a vector opposite to the swing direction and his left hand holds it with a vector in the swing direction. As compared to other sport equipment such as a baseball bat, a tennis racket, and the like, a golf club has an unnatural shape with respect to weight distribution. Such a shape of the golf club makes it difficult for a golf player to perform a perfect golf swing and to retain precise control of the swing path.
The above-mentioned feeling during swing can be easily demonstrated such that shadow swing is performed while the toe side of the club head faces forward or vise versa.